It has long been known in the thermostat art that creep action is to be avoided. The primary reason for the avoiding of creep action has been to prevent damage to the contacts caused by arcing as the contacts just begin to separate, and to prevent other undue wear on these contacts caused by arcing and chattering.
A solution adopted by the prior art to avoid creep action has been the use of the snap-action thermostat. In particular, this is accomplished by the formation of a dimple in a bimetallic member, the dimple snapping from a convex to a concave, or concave to convex, shape when the preset temperature is reached. This snapping of the dimple causes a rapid movement of the bimetallic arm in which the dimple is formed, resulting in a sudden separation of the two contacts in the thermostat. Thus, creep is avoided.
The use of snap-action thermostats, as described, however is not entirely free from problems. In particular, thermostats formed with snap-action members are about twice as expensive as those formed with creep action members. Further, when formed in automatic assembly equipment, as an extremely large number of thermostats are today, there is a substantial loss of thermostats employing the snap-action, dimpled bimetallic arms. Frequently, as many as 50% of the snap-action arms are lost in testing following formation; by comparison, there is generally about a 96% yield of creep action type thermostats.
Attempts have been made by the industry to obtain the benefit of the snap-action type of system, desired for its operation and equipment wear, while utilizing, essentially, a creep action type of thermostat. In general, however, these have been hybrid systems, such as shown in U.S. Pat. Nos. 3,789,339, 3,851,288, and particularly 4,319,214, all assigned to the assignee of the present invention. While each of these thermostats provides an adequate solution for the problems it is to solve, it does not really provide for a thermostat giving the benefits of snap action, while avoiding the costs and assembly losses experienced with that type of thermostat.
In addition, recently, an increased emphasis has been placed on thermal protectors for incandescent light fixtures. Because of the nature of the service, there is a high initial inrush of current, as the incandescent fixture is illuminated, followed, quite rapidly, by a substantial drop in current. If the thermostat employed is not able to accept this initial surge without breaking the circuit, it is difficult, if not impossible, to illuminate the incandescent fixture. Accordingly, the industry has also sought a thermostat for an incandescent fixture which will accommodate the high initial current surges without breaking the electric circuit, while still providing sufficient protection to interrupt the circuit should problems develop during regular operation of the incandescent fixture.